PICALM: Role in the pathogenesis and treatment of Alzheimer vascular blood-brain barrier clearance dysfunction, neuronal dysfunction, and amyloid-beta, tau and neurodegenerative disorders

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PICALM is one of the most significant susceptibility factors for late onset Alzheimer’s disease (LOAD). Its role in disease pathogenesis, however, remains elusive. We also do not have an effective PICALM-based therapy for AD. PICALM controls internalization of cell receptors, and intracellular trafficking of different proteins. PICALM is abundantly expressed in brain endothelium and neurons, but is reduced in LOAD and by some PICALM SNPs. To understand how PICALM regulates vascular and neuronal function and AD pathology, we developed new mouse models with PICALM-specific deletion from endothelium and neurons. The proposed studies are supported by our pilot data showing: i) that PICALM controls amyloid-β (Aβ) and tau clearance across the blood- brain barrier (BBB) and guides their trans-endothelial BBB transcytosis, and that PICALM endothelial deficiency leads to Aβ and tau brain accumulation; and ii) that PICALM loss from neurons leads to neuron loss, and renders them susceptible to both excitotoxic injury due to N-methyl-D-aspartate receptors (NMDAR) overexpression, and elevated Aβ and tau toxicity resulting from diminished PICALM binding to, and sequestration of glucose regulated protein 78 (GRP78) in the cytosol. This in turn shuttles free GRP78 to endoplasmic reticulum (ER) hampering unfolded protein response (UPR) which aggravates ER stress response to Aβ and tau. Since PICALM deficiency leads to loss-of-function, we propose to test therapies to increase PICALM with artesunate, a lead drug from our pilot FDA-approved library screen, and with gene therapy. We also generated a new Picalm465R line carrying a rare 465R PICALM missense mutation that does not alter PICALM expression, but increases its binding to GRP78 in neurons and LRP1 in endothelium. Based on our pilot data, we hypothesize that PICALM endothelial deficiency will lead to Aβ and tau brain accumulation due to their impaired clearance at the BBB caused by loss of PICALM binding to LRP1 and its deficient interactions with Rab5 and Rab11 during PICALM-guided Aβ and tau BBB transcytosis; whereas PICALM neuronal deficiency will render neurons susceptible to excitotoxic injury due to NMDAR overexpression, and will increase Aβ and tau neuronal toxicity by increasing GRP78 translocation from the cytosol to ER that will hamper UPR and augment ER stress response to Aβ and tau. Therapies to increase PICALM, and 465R mutant with enhanced binding to LRP1 and GRP78, will increase Aβ and tau BBB clearance and protect neurons. We will study the effects of endothelium-specific (AIM 1) and neuron-specific (AIM 2) PICALM deficiency on vascular and neuronal function and AD pathology; and the effects of artesunate and AAV-PHP.B-Picalm gene therapy (AIM 3), and the H465R PICALM mutation (AIM 4) on vascular and neuronal function and AD pathology. We will next identify molecular steps in Aβ and tau BBB transcytosis and neuronal toxicity regulated by PICALM using BBB models and neurons from human rs3851179 PICALM variants and Picalm465R mice (AIM 5). If successful, this proposal will generate unique new insights into PICALM biology with implications for better understanding of the role of PICALM in the pathogenesis and treatment of AD.